Shape memory alloys (SMA) form a group of metals that have interesting thermal and mechanical properties. If a SMA material such as NiTinol is deformed while in a martensitic state (low yield strength condition) and then heated to its transition temperature to reach an austenitic state, the SMA material will resume its original (undeformed) shape. The rate of return to the original shape depends upon the amount and rate of thermal energy applied to the component. When the SMA material is cooled, it will return to the martensitic state and shape. Properties of and information about shape memory alloys can be found at http://www.sma-inc.com/SMAPaper.html or http://www.SMA-mems.com/info.html, which we also incorporate by reference.
The application of ‘smart structures’ to helicopter rotors has received widespread study in recent years, and is one thrust of the Shape Memory Alloy Consortium (SMAC) program, which Boeing leads. The SMAC program includes NiTinol fatigue/characterization studies, SMA actuator development, and ferromagnetic SMA material development (offering increased actuation speed). An SMA torsional (i.e. rotary) actuator of the present invention for rotocraft (i.e., helicopters or tilt rotors) retwists a rotocraft blade in flight, and results in a significant payload gain for the vehicle.
Distributed fibers of piezo material embedded in a composite blade can accomplish dynamic twisting. Twists of a degree or two are adequate to achieve dynamic control of vibration and acoustics. By laying the piezo fibers at ±45° to the blade axis and actuating the piezo material along the fiber direction, the piezo strain twists the blade about 2°, either dynamically at control frequencies (for vibration & noise reduction) or statically (for some payload increase). Implementing this technology requires high quality, low cost piezo fibers and high voltage, high power efficient drive amplifiers.
A second method of dynamic control uses a flap, actuated by a piezo stack actuator mounted on the rotor spar. A few degrees of flap motion provide adequate dynamic control of the rotor blade. This method also achieves vibration or acoustic benefits.
One key to getting good response out of an SMA such as NiTinol is to have a good cooling path. The NiTinol needs to be kept as thin as possible, consistent with the load requirements. In one actuator design, we surrounded a NiTinol torque tube with a thin brass tube wrapped with Nichrome foil heater tape. We initially wound the heater tape directly on the torque tube, but discovered that the tape could not stand the large torsion actuation strain the tube undergoes. The axial windings of the tape were nonuniform to help keep the tube at a constant, desired temperature, but the attachments for the tube made it difficult to achieve even, constant heating. The housing carries the heat load from the passive torque tube when the actuator was unpowered. Small air gaps within the actuator impose significant thermal resistance, and grease was required between the NiTinol/brass and heater/housing. The performance measured for the device suggests that 500 watts of power are required for a 20-second response as a nominal value of the power requirement for a ⅙-scale actuator. Scaling laws based upon our tests indicate that simply scaling this design (to full scale) would result in a heavy actuator requiring large power to achieve a constant response time, or, alternatively, would sacrifice response time at reasonable power requirements.
U.S. Pat. No. 5,127,228 describes a ‘smart structure’ actuator device having an inner SMA (e.g., NiTinol) torque tube seated concentrically within an outer SMA torque tube. Ends of both tubes are mechanically restrained to an indexed position. Because one tube provides torque clockwise while the other tube provides counter-clockwise torque, the tubes are arranged in an opposing manner. Initially, both SMA members are in a martensitic state. A power supply supplies current to heaters that are connected to one of the tubes to control switching of the SMA between memory states. Heating causes rotation of the actuator in a clockwise or counter-clockwise rotational direction, as desired. The electrical energy passing through the heater(s) causes the SMA to which the heater is connected to transition from its martensitic to austenitic state, resulting in the rotation. Control of the electrical power to the heaters allows holding the actuator in a selected rotational position or allows rotation in either rotational direction.
To maintain a specific rotational position in a loaded condition, the '228 device requires continuous electrical power to the heater elements for both SMA tubes. This shortcoming of the '228 device adds significant system weight and complexity and requires excessive power. The '228 device requires the addition of thermal insulation to isolate the tubes thermally or a skillful design so that the heater for one tube does not heat the wrong tube and, thereby, unintentionally create an actuator malfunction.
Other known SMA rotary actuators use a single SMA member to produce the desired reciprocating rotation at desired intervals. These devices use the SMA member to provide rotation in one direction, while using a mechanical spring, a flexure, or another suitable restorative device to provide rotation of the actuator in the return direction. The force achievable with mechanical springs is limited. Large springs having adequate force add considerable weight and mass to the actuator mechanism, which lessens performance and restricts their implementation. The mechanical springs also deteriorate over time, which limits the reliability of the actuator.
A need exists for a SMA rotary actuator to provide any or all of the following properties: (1) controllable torque either in low or high amount, (2) operation in both rotational direction using switching of the state of a single SMA member, locking at a desired rotational position without requiring constant supply of power to heaters associated with the SMA members, or (3) return rotation without applying electrical power. Such a rotary actuator should also be capable of generating a significant torque over a large angle of rotation. A small size and low weight also is beneficial for an improved device. The actuator of the present invention addresses these needs.